Abstract: A belt-pulley drive mechanism comprising a pulley having a plurality of circumferentially extending grooves of V-shaped cross-sectional configuration, a belt adapted to be passed around the pulley and stretched under tension so that the belt is engaged with the pulley in torque transmitting relationship, the belt being formed with a plurality of longitudinally extending ridges of V-shaped cross-sectional configuration which are adapted to be engaged with the grooves in the pulley when the tension is applied to the belt. The grooves or the ridges or both are formed in a cross-section along a concave line so that a clearance is formed between the pulley and the belt when the tension is removed from the belt, the clearance being larger at a transversely central portion than at side edge portions.